This invention relates generally to information delivery systems, and more particularly to delivery systems that adapt information encoded as compressed bitstreams to available bit rates of a network and to semantic content of the bitstream.
Recently, a number of standards have been developed for communicating encoded information. For digital images, the best known standard is JPEG, see Pennebacker et al., in xe2x80x9cJPEG Still Image Compression Standard,xe2x80x9d Van Nostrand Reinhold, 1993. For video sequences, the most widely used standards include MPEG-1 (for storage and retrieval of moving pictures), MPEG-2 (for digital television) and H.263, see ISO/IEC JTC1 CD 11172, MPEG, xe2x80x9cInformation Technologyxe2x80x94Coding of Moving Pictures and Associated Audio for Digital Storage Media up to about 1.5 Mbit/sxe2x80x94Part 2: Coding of Moving Pictures Information,xe2x80x9d 1991, LeGall, xe2x80x9cMPEG: A Video Compression Standard for Multimedia Applications,xe2x80x9d Communications of the ACM, Vol. 34, No. 4, pp. 46-58, 1991, ISO/IEC DIS 13818-2, MPEG-2, xe2x80x9cInformation Technologyxe2x80x94Generic Coding of Moving Pictures and Associated Audio Informationxe2x80x94Part 2: Video,xe2x80x9d 1994, ITU-T SG XV, DRAFT H.263, xe2x80x9cVideo Coding for Low Bitrate Communication,xe2x80x9d 1996, ITU-T SG XVI, DRAFT13 H.263+Q15-A-60 rev.0, xe2x80x9cVideo Coding for Low Bitrate Communication,xe2x80x9d 1997.
These standards are relatively low-level specifications that primarily deal with the spatial compression of images and the spatial and temporal compression of video sequences. As a common feature, these standards perform compression on a per frame basis. With these standards, one can achieve high compression ratios for a wide range of applications.
Newer video coding standards, such as MPEG-4 (for multimedia applications), see xe2x80x9cInformation Technologyxe2x80x94Generic coding of audio/visual objects,xe2x80x9d ISO/IEC FDIS 14496-2 (MPEG4 Visual), Nov. 1998, allow arbitrary-shaped objects to be encoded and decoded as separate video object planes (VOP). The objects can be visual, audio, natural, synthetic, primitive, compound, or combinations thereof.
The emerging MPEG-4 standard is intended to enable multimedia applications, such as interactive video, where natural and synthetic materials are integrated, and where access is universal. For example, one might want to xe2x80x9ccut-and-pastexe2x80x9d a moving figure or object from one video to another. In this type of application, it is assumed that the objects in the multimedia content have been identified through some type of segmentation process, see for example, U.S. patent application Ser. No. 09/326,750 xe2x80x9cMethod for Ordering Image Spaces to Search for Object Surfacesxe2x80x9d filed on Jun. 4, 1999 by Lin et al.
In the context of video transmission, these compression standards are needed to reduce the amount of bandwidth (available bit rate) that is required by the network. The network may represent a wireless channel or the Internet. In any case, the network has limited capacity and a contention for its resources must be resolved when the content needs to be transmitted.
Over the years, a great deal of effort has been placed on architectures and processes that enable devices to transmit the content robustly and to adapt the quality of the content to the available network resources. When the content has already been encoded, it is sometimes necessary to further compress the already compressed bitstream before the stream is transmitted through the network to accommodate a reduction in the available bit rate.
As shown in FIG. 1, typically, this can be accomplished by a transcoder 100. In a brute force case, the transcoder includes a decoder 110 and encoder 120. A compressed input bitstream 101 is fully decoded at an input rate Rin, then encoded at a new output rate Rout 102 to produce the output bitstream 103. Usually, the output rate is lower than the input rate. However, in practice, full decoding and full encoding in a transcoder is not done due to the high complexity of encoding the decoded bitstream.
Earlier work on MPEG-2 transcoding has been published by Sun et al., in xe2x80x9cArchitectures for MPEG compressed bitstream scaling,xe2x80x9d IEEE Transactions on Circuits and Systems for Video Technology, April 1996. There, four methods of rate reduction, with varying complexity and architecture, were presented.
FIG. 2 shows an example method. In this architecture, the video bitstream is only partially decoded. More specifically, macroblocks of the input bitstream 201 are variable-length decoded (VLD) 210. The input bitstream is also delayed 220 and inverse quantized (IQ) 230 to yield discrete cosine transform (DCT) coefficients. Given the desired output bit rate, the partially decoded data are analyzed 240 and a new set of quantizers is applied at 250 to the DCT blocks. These re-quantized blocks are then variable-length coded (VLC) 260 and a new output bitstream 203 at a lower rate can be formed. This scheme is much simpler than the scheme shown in FIG. 1 because the motion vectors are re-used and an inverse DCT operation is not needed.
More recent work by Assuncao et al., in xe2x80x9cA frequency domain video transcoder for dynamic bit-rate reduction of MPEG-2 bitstreams,xe2x80x9d IEEE Transactions on Circuits and Systems for Video Technology, pp. 953-957, December 1998, describe a simplified architecture for the same task. They use a motion compensation (MC) loop, operating in the frequency domain for drift compensation. Approximate matrices are derived for fast computation of the MC blocks in the frequency domain. A Lagrangian optimization is used to calculate the best quantiser scales for transcoding.
Other work by Sorial et al, xe2x80x9cJoint transcoding of multiple MPEG video bitstreams,xe2x80x9d Proceedings of the International Symposium on Circuits and Systems, May 1999, presents a method of jointly transcoding multiple MPEG-2 bitstreams, see also U.S. patent application Ser. No. 09/410,552 xe2x80x9cEstimating Rate-Distortion Characteristics of Binary Shape Data,xe2x80x9d filed Oct. 1, 1999 by Vetro et al.
According to prior art compression standards, the number of bits allocated for encoding texture information is controlled by a quantization parameter (QP). The above papers are similar in that changing the QP based on information that is contained in the original bitstream reduces the rate of texture bits. For an efficient implementation, the information is usually extracted directly in the compressed domain and may include measures that relate to the motion of macroblocks or residual energy of DCT blocks. This type of analysis can be found in the bit allocation analyzer.
Although in some cases, the bitstream can be preprocessed, it is still important that the transcoder operates in real-time. Therefore, significant processing delays on the bitstream cannot be tolerated. For example, it would not be feasible for the transcoder to extract information from a group of frames, then transcode the content based on this look-ahead information. This would not work for live broadcasts, or video conferencing. Although it is possible to achieve better transcoding results in terms of quality due to better bit allocation, such an implementation for real-time applications is impractical.
It is also important to note that classical methods of transcoding are limited in their ability to reduce the bit rate. In other words, if only the QP of the outgoing video is changed, then there is a limit to how much one may reduce the rate. The limitation in reduction is dependent on the bitstream under consideration. Changing the QP to a maximum value will usually degrade the content of the bitstream significantly. Another alternative to reducing the spatial quality is to reduce the temporal quality, i.e., drop or skip frames. Again, skipping too many frames will also degrade the quality significantly. If both reductions are considered, then the transcoder is faced with a trade-off in spatial versus temporal quality. However, even with such reduction in spatial and temporal resolution, it may be difficult for the transcoder to meet its target rate without destroying the content that is conveyed by the original video bitstream.
As a result, the transcoder must find some alternate means of transmitting the information that is contained in a bitstream to adapt to reductions in available bit rates.
The invention provides a multi-media delivery system for delivering a compressed bitstream through a network to a user device. The system includes a transcoder and a manager. The transcoder is configured to operate on the bit stream using any one of a plurality of conversion modes. The manager is configured to select a particular one of the plurality of conversion modes depending on semantic content of the bitstream and network characteristics. The system also includes a content classifier to determine the content characteristics, and a model predicator to determine the network characteristics, and user device characteristics. An integrator of the manager generates optimal rate-quality functions to be used for selecting the particular conversion model for a given available bit rate of the network.